1. Field of the Invention
The present invention relates to contactless receivers, resonant circuits, and variable capacitance elements, more specifically, to a contactless receiver, a resonant circuit, and a variable capacitance element which are suitably applied to devices, such as a contactless IC card, which receive electric waves from a reader/writer.
2. Description of the Related Art
Variable capacitance elements which receive bias signals from outside to change their capacitances to control frequency, time, etc. have been used. Examples of such variable capacitance elements which have been introduced into the market include variable capacitance diodes (varicaps) and MEMS (Micro Electro Mechanical Systems).
FIGS. 11A and 11B show a circuit configuration and an example of characteristics of a variable capacitance element. FIG. 11A shows a circuit configuration surrounding a variable capacitance element 160, and FIG. 11B shows a dependence of the capacitance Cv of the variable capacitance element 160 on control voltages. The variable capacitance element 160 normally has two terminals, and has no dedicated terminal for receiving bias signals to control the capacitance. Therefore, the variable capacitance element 160 is configured to have 4 terminals on an actual circuit as shown in FIG. 11A. More specifically, one terminal of the variable capacitance element 160 (variable capacitor) is connected to one input output terminal for alternating current signals via a bias removal capacitor 161, and is also connected to an input terminal for control voltage signals via a current limiting resistor 162. In addition, the other terminal of the variable capacitance element 160 is connected to the other input output terminal for the alternating current signals, and is also connected to an output terminal for the control voltage signals.
In a circuit configuration of the variable capacitance element 160 as described above, signal currents (alternating current signals) flow through the bias removal capacitor 161 and the variable capacitor 160, and control currents (DC bias current) flow only through the variable capacitor 160 via the current limiting resistor 162. In this process, the capacitance Cv of the variable capacitor 160 is changed by changing the control voltages, resulting in a change of the signal currents as shown in FIG. 11B.
Technologies for using variable capacitance elements as described above as a protection circuit in contactless IC cards have been proposed (refer to, for example, Japanese Unexamined Patent Application Publication No. 2008-7059). In Japanese Unexamined Patent Application Publication No. 2008-7059, variable capacitance elements are used as a protection circuit to avoid destruction of a control circuit including semiconductor devices with a low voltage endurance caused by excessively strong received signals when the contactless IC cards are brought close to the reader/writer (hereinafter referred to as an R/W).
FIG. 12 is a block diagram of the contactless IC cards proposed in Japanese Unexamined Patent Application Publication No. 2008-7059. In Japanese Unexamined Patent Application Publication No. 2008-7059, a variable capacitance diode 103d is used as a variable capacitance element. In addition, a series circuit including a bias removal capacitor 103c and the variable capacitance diode 103d is connected in parallel to a resonant circuit including a coil 103a and a capacitor 103b. 
In Japanese Unexamined Patent Application Publication No. 2008-7059, a direct current voltage Vout obtained by detecting the received signals at a detector circuit 113 is divided by resistors 114a and 114b. The divided direct current voltage (direct current voltage applied to the resistor 114b) is applied to the variable capacitance diode 103d via a coil 115 to remove its fluctuation in order to adjust the capacitance of the variable capacitance diode 103d. That is, the divided direct current voltage is used as a control voltage of the variable capacitance diode 103d. 
According to Japanese Unexamined Patent Application Publication No. 2008-7059, when there are excessively strong received signals, the capacitance of the variable capacitance diode 103d becomes smaller with the control voltage, making a resonance frequency of a receiving antenna 103 higher. FIG. 13 shows characteristics of the resonance frequency. In the characteristics shown in FIG. 13, the horizontal axis indicates frequencies, and the vertical axis indicates responses of signals. When there are excessively strong received signals, the capacitance of the variable capacitance diode 103d becomes smaller (that is, the combined capacitance of the receiving antenna 103 becomes smaller), and the resonance frequency of the receiving antenna 103 shifts to the higher region by the frequency Δf corresponding to the amount of the decrease in capacitance (as shown by the dashed line in FIG. 13). This results in a response of the received signals at a frequency f0 lower than that before the capacitance decreased, allowing the control of the level of the received signals. The technology proposed in Japanese Unexamined Patent Application Publication No. 2008-7059 protects a control circuit 120 with a variable capacitance element as described above.
The inventors have also proposed elements using ferroelectric material as a variable capacitance element (for example, refer to Japanese Unexamined Patent Application Publication No. 2007-287996). In Japanese Unexamined Patent Application Publication No. 2007-287996, a variable capacitance element 200 with an electrode structure as shown in FIGS. 14A and 14B has been proposed for higher reliability and productivity. FIG. 14A is a perspective view showing the outline of the variable capacitance element 200, and FIG. 14B is a sectional view showing the configuration of the variable capacitance element 200. The variable capacitance element 200 according to Japanese Unexamined Patent Application Publication No. 2007-287996 is provided with a terminal at each of four surfaces of a cubic-shaped dielectric layer 204. Two terminals facing each other among the four terminals are signal terminals 203a and 203b connected to a signal power supply 203, and the other two terminals facing each other are control terminals 202a and 202b connected to a control power supply 202.
Inside the variable capacitance element 200, as shown in FIG. 14B, a plurality of control electrodes 202c to 202g and a plurality of signal electrodes 203c to 203f are laminated one over another within a ferroelectric layer 204. In the example of FIG. 14B, the control electrode 202g at the lowest layer, the control electrode 202e at the 5th layer from the bottom, and the control electrode 202c at the top layer are connected to the control terminal 202a at one side. The control electrode 202f at the 3rd layer from the bottom and the control electrode 202d at the 7th layer from the bottom are connected to the control terminal 202b at the other side. In addition, the signal electrode 203f at the 2nd layer from the bottom and the signal electrode 203d at the 6th layer from the bottom are connected to the signal terminal 203a at one side. Moreover, the signal electrode 203e at the 4th layer from the bottom and the signal electrode 203c at the 8th layer from the bottom are connected to the signal terminal 203b at the other side.
The variable capacitance element 200 disclosed in Japanese Unexamined Patent Application Publication No. 2007-287996 provides advantages of being able to individually apply voltages to the control terminal and the signal terminal, and of being able to increase the capacitance with low cost by laminating layers of signal electrodes and control electrodes inside. In addition, the variable capacitance element 200 having a structure as disclosed in Japanese Unexamined Patent Application Publication No. 2007-287996 is easy to manufacture and has a low cost. Moreover, the variable capacitance element 200 according to Japanese Unexamined Patent Application Publication No. 2007-287996 does not need a bias removal capacitor 161 shown in FIG. 11A.